Problem: Which of these numbers is composite? ${7,\ 13,\ 17,\ 29,\ 39}$
Answer: A composite number is a number that has more than two factors (including 1 and itself). 7, 13, 17, and 29 each have only two factors. The factors of 39 are 1, 3, 13, and 39. Thus, 39 is the composite number.